Light Emitting Diodes (LEDs) are increasingly being adopted as general illumination lighting sources due to their high energy efficiency and long service life relative to traditional sources of light such as incandescent, fluorescent and halogen. Each generation of LEDs are providing improvements in energy efficiency and cost per lumen, thus allowing for lighting manufacturers to produce LED light fixtures at increasingly cost competitive prices. One key differentiator for LEDs over the traditional sources of light is their ability to provide high quality light with varying wavelengths based on the user's desires.
Typical LEDs today are made from a variety of inorganic semiconductor materials and can either be focused to a specific limited range of output wavelengths of light or can be made to have a broad spectrum of output wavelengths. Table 1 below summarizes a sampling of LED color options, the wavelengths for those colors and the material that they can be produced with:
TABLE 1ColorWavelength (nm)Semiconductor MaterialInfrared>760GaAs, AlGaAsRed610-760AlGaAs, GaAsP, AlGaInP, GaPOrange590-610GaAsP, AlGaInP, GaPYellow570-590GaAsP, AlGaInP, GaPGreen500-570InGaN/GaN, GaP, AlGaInP, AlGaPBlue450-500ZnSe, InGaN, SiC as substrateViolet400-450InGaNUltraviolet<400AlN, AlGaN, AlGaInN, diamond, BNWhitebroad spectrumBlue or UV LED with yellow phosphor
The above table is not meant to be a complete list but rather to illustrate the wide range in color varieties and various different semiconductor materials that have been used to-date. For example, there are new phosphor coated LEDs on the marketplace that allow for wavelength shifting of various LEDs (ex. phosphor shifted Amber LEDs).
LEDs provide opportunities to offer users a wide variety of light outputs due to the various wavelengths that can be produced. In some LED light fixtures, Red, Green, Blue (RGB) or Red, Green, Blue, Amber (RGBA) combinations are used to create white light. In some embodiments of these light fixtures, integrated LED modules are used that include LEDs of all three or four colors of light. In this case, one or more of these RGB/RGBA modules are coupled in series to generate the desired white light output. In other embodiments, a plurality of strings of single color LEDs of varying colors are used, each string of LEDs being controlled simultaneously. In some cases, these strings of LEDs may be controlled independently with separate Pulse Width Modulation (PWM) signals that dictate the length of time during a duty cycle that each string of LEDs are in operation (the “on time”). In these embodiments, a controller for the light fixture can select a variety of different light outputs by adjusting the “on time” for the various strings of LEDs.
Although the use of RGB/RGBA architectures enable for a white light output with varying colors, the light output is actually a diffused version of three or four individual wavelengths of light. These lights do not provide a full spectrum white light and normally cannot provide a high Color Rendering Index (CRI). CRI is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source being a measure of light quality. Similarly, white LEDs (phosphor coated blue or UV LEDs) provide a white light output but do not provide a full spectrum and typically have a significant gap around 500 nm. Typical white LEDs also do not rate highly on CRI.
LEDs are expensive on a cost per lumen basis and can constitute a large portion of the costs for an LED light fixture. Lighting manufacturers are often working to minimize the number of LEDs within their LED light fixtures while still maintaining the desired light output in intensity and color/color temperature. When it is desired to provide the user of the light the ability to adjust the colors or color temperatures of white, a lighting manufacturer may use large numbers of LEDs creating a plurality of strings of LEDs in series.
When designing LED light fixtures for small spaces or within small traditional lighting designs (ex. MR16), the amount of LEDs used may be physically constrained by the situation. In these cases, the options for varying color or color temperature may be limited as the space to implement a plurality of strings of LEDs may not be available.
Against this background, there is a need for solutions that will enable varying light outputs within LED lighting apparatus while reducing the quantity of LEDs required.